Scalp Derivations Research Articles

Motor neuron disease with pyramidal tract dysfunction involves the cortical generators of the early somatosensory evoked potential to tibial nerve stimulation.

We evaluated somatosensory evoked potentials (SEPs) to tibial nerve stimulation in 39 patients with sporadic motor neuron disease using multiple scalp derivations (earlobe reference). SEPs were altered in 22 of 29 amyotrophic lateral sclerosis (ALS) patients, whereas they were unaffected in 10 progressive muscular atrophy (PMA) patients. The main changes involved the amplitude and the field distribution of the early P40 and N37 cortical potentials with different modalities varying from a selective loss of the P40 potential (33% of tested sides) to absence of all early cortical SEPs (22% of tested sides). The later components following N50 were generally spared. The commonly used Cz-Fz montage was inadequate for detecting these alterations. Central afferent conduction was slightly affected. The selective loss of cortical SEPs and their close correlation with clinicoelectrophysiologic evidence of central motor system involvement strongly support a cortical origin of the SEP alterations in ALS. We suggest that neuronal loss in the somatosensory cortex may selectively affect the generator sites of the cortical SEPs to lower limb stimulation.

Reappearance of event-related P3 potential in locked-in syndrome

In a patient affected by locked-in syndrome, because of a lesion of the caudal-ventral pontine tegment, we recorded event-related potentials (ERPs) to an acoustic odd-ball paradigm. We did not record N2 and P3 components in the 50 days following the acute lesion although the patient was able to understand the task and communicate with blinking. 55 days after the acute lesion, N2 and P3 reappeared on scalp derivations. This findings suggest that brainstem structures are involved in P3 or N2-P3 generation.

Reappearance of event-related P3 potential in locked-in syndrome

In a patient affected by locked-in syndrome, because of a lesion of the caudal-ventral pontine tegment, we recorded event-related potentials (ERPs) to an acoustic odd-ball paradigm. We did not record N2 and P3 components in the 50 days following the acute lesion although the patient was able to understand the task and communicate with blinking. 55 days after the acute lesion, N2 and P3 reappeared on scalp derivations. This findings suggest that brainstem structures are involved in P3 or N2-P3 generation.

Source model and scalp topography of pattern reversal visual evoked potentials to altitudinal stimuli suggest that infoldings of calcarine fissure are not part of VEP generators.

Visual evoked potentials (VEPs) to pattern reversal vertical bar stimuli were recorded from 19 scalp, 2 zygomatic and 3 inion derivations referenced to digitally linked earlobes in 50 controls. 1, 2 and 4 cycles per degree (cpd) patterns were presented as full field (FF) stimuli, on upper and lower hemifields (UHF-LHF), upper and lower quadrants and with the occlusion of central and peripheral UHF and LHF. VEPs to octant stimuli were also recorded with 2 cpd patterns. N1, P1 and N2 components were recorded from posterior and inion derivations with FF stimuli, from posterior derivations with LHF stimuli, only from inion leads with UHF stimuli, from derivations ipsilateral to stimuli with quadrants and octants, and from midline derivations only with lower quadrants. Polarity inverted sequences (iP1-iN1-iP2) were recorded from the other scalp derivations, with similar latency and spatial frequency sensitivity as N1-P1-N2. The orientation of Equivalent Dipoles (ED) was orthogonal with surface coordinates of mesial and occipito-polar calcarine cortex, measured on Magnetic Resonance Imaging. A model of VEP generators is proposed, suggesting that the VEP sequence is elicited only in mesial and occipito-polar surfaces of calcarine cortex.

Visual evoked potentials generator model derived from different spatial frequency stimuli of visual field regions and magnetic resonance imaging coordinates of V1, V2, V3 areas in man.

Visual evoked potentials (VEPs) to pattern reversal vertical bar stimuli were recorded from 24 scalp derivations (including zygomatic and inion) referenced to digitally linked earlobes in 50 controls. 1, 2 and 4 cpd patterns were presented as full field (FF) stimuli, on Upper Hemifields (UHF) and Lower Hemifields (LHF), upper and lower quadrants and with the occlusion of central and peripheral UHF and LHF. VEPs to octant stimuli were also recorded with 2 cpd patterns. N1, P1 and N2 components were recorded from posterior and inion derivations with FF stimuli, from posterior derivations with LHF stimuli, only from inion leads with UHF stimuli, from derivations ipsilateral to stimuli with quadrants and octants, and consistently from midline derivations only with lower quadrants. Polarity inverted sequences (iN1-iP1-iN2) were recorded from the other scalp derivations, with similar latency and spatial frequency sensitivity as N1-P1-N2. Single Equivalent Dipole (ED) calculations were performed on N1 and P1 recorded in the different stimulus conditions. Our findings contradict previous hypotheses on VEP generators and contradict the predictions of VEPs polarity and distribution based on the "cruciform model" of VEPs generators. In order to explain the distribution of VEPs to upper and lower half fields and to quadrant and octants, we propose a model based on the position of the medial and occipito-polar surface of visual cortex in man.

P3 recordings in patients with bilateral temporal lobe lesions.

We recorded event-related potentials to an acoustic "oddball" paradigm from 19 scalp derivations in five patients (three women and two men; age range, 44 to 56) who had global amnesia following encephalitis. CT and MRI showed severe bilateral lesions of anterior and midtemporal lobes. A P3 component, with a peak latency within the normal limits for age-matched controls, was recorded to "target" stimuli in four patients from all leads except Fp1, Fp2, F7, F8, T3, and T4. In the fifth patient, the P3 peak was delayed.

Reliability of topographic quantitative EEG amplitude in healthy late-middle-aged and elderly subjects

Reliabilities of quantitative measures of absolute and relative EEG amplitudes were assessed in healthy older adults under the eyes closed (n = 46) and eyes opened (n = 45) conditions. For the theta, alpha, beta 1, and beta 2 bands, reliabilities of 28 scalp derivations were stable over the 4.5 month test interval. Reliabilities of delta were lower. When appropriate transformations were applied, the reliabilities of absolute EEG amplitude measures tended to exceed those of relative measures. There were not, however, striking differences in reliabilities under the eyes closed, as compared to eyes opened condition. We concluded that when coupled with the criterion of interpretability, the generally higher reliabilities of absolute, as opposed to relative, amplitude measures render them preferable in clinical research.

Persistence of a P3 component in severe amnestic syndrome

Event-related potentials (ERPs) to an odd-ball paradigm were recorded from 19 scalp derivations in a patient (47 years old, housewife) affected by complete amnestic syndrome following limbic encephalitis. CT scans and magnetic resonance imaging showed severe bilateral lesions of anterior, mid-temporal lobes and frontal lobes. A P3 component, with a peak latency inside normal limits for age-matched controls, was recorded to ‘target’ stimuli from all leads except Fp1, Fp2, F7, F8, T3 and T4.

Mapped distribution of pattern reversal VEPs to central field and lateral half-field stimuli of different spatial frequencies

Visual evoked potentials (VEPs) to pattern reversal vertical bar stimuli of 3 different sizes (1, 2, 4 c/deg) were recorded from 19 scalp derivations in 50 controls. The stimuli were presented on a full-field (FF) screen of 24° visual angle, and on left and right half-fields (HF) of 12° radius. In 15 controls partial HF stimuli were presented on the central 3 and 6° and as hemiannular stimuli of 12° with occlusion of the central 3 and 6°. An antero-posterior polarity reversal of the N1-P1-N2 sequence was observed for FF VEPs. A tangential polarity reversal was observed for HF VEPs. Also with central or hemiannular stimuli polarity reversals of all VEP components were observed within the scalp. Variants of VEP distribution, absence of prominence of some of the ipsi- or contralateral VEP components were observed in 8–40% of controls. The FF and HF VEP distribution, and the variant VEP asymmetries were partly dependent on the pattern spatial frequency.

EEG amplitudes in healthy, late-middle-aged and elderly adults: normality of the distributions and correlations with age

The interindividual distributions of EEG amplitudes were evaluated in 60 healthy subjects, aged 56–76. Skew and kurtosis were used to assess the normality of the amplitude distributions in 5 frequency bands (delta, theta, alpha and 2 beta ranges) and 28 scalp derivations while the subjects were awake and rested with their eyes closed and opened. Most of the interindividual EEG amplitude distributions were not normally distributed. Two transformations were applied to the EEG amplitudes: the square root and the natural logarithm. Evaluation of skew and kurtosis indicated that the natural logarithmic transformation was more effective than the square root in reducing the positive skew and leptokurtosis that characterized the untransformed interindividual EEG amplitude distributions. For all frequency bands except theta, the log transformation rendered the distributions approximately normal in form. Correlations between log transformed EEG amplitudes and agewere not statistically significant. These findings are discussed in the context of previous research, aging, and methodological features.

Far‐field and cortical somatosensory evoked potentials in motor neuron disease

We examined median somatosensory evoked potentials (SEPs) in 26 patients with sporadic motor neuron disease (MND). SEPs were recorded with multiple scalp derivations, using both the midfront and the earlobe as references for each subject. Central conduction time (CCT) was abnormal in three patients, but only when using the midfront reference. Moreover, an exclusive alteration of the early prerolandic potentials (absent or delayed P20 and/or P22) was noted using the earlobe reference in amyotrophic lateral sclerosis and in progressive bulbar palsy (54% and 50% of patients, respectively) but not in progressive muscular atrophy. These findings correlated with clinical evidence of upper motor neuron signs and with the severity of the disease. In agreement with recent views regarding the sources of the early anterior cortical responses, neuronal loss in the motor cortex may be considered as affecting the generator sites of these potentials.

Wave analysis on surface oscillation signals from twitch contraction

Our aim was to enhance the spontaneous slow-frequency EEG activity during the resting state using oscillating transcranial direct currents (tDCS) with a stimulation frequency that resembles the spontaneous oscillations of sleep onset. Accordingly, in this preliminary study, we assessed EEG after-effects of a frontal oscillatory tDCS with different frequency (0.8 vs. 5 Hz) and polarity (anodal, cathodal, and sham).Two single-blind experiments compared the after effects on the resting EEG of oscillatory tDCS [Exp. 1 = 0.8 Hz, 10 subjects (26.2 ± 2.5 years); Exp. 2 = 5 Hz, 10 subjects (27.4 ± 2.4 years)] by manipulating its polarity.EEG signals recorded (28 scalp derivations) before and after stimulation [slow oscillations (0.5–1 Hz), delta (1–4 Hz), theta (5–7 Hz), alpha (8–12 Hz), beta 1 (13–15 Hz) and beta 2 (16–24 Hz)] were compared between conditions as a function of polarity (anodal vs. cathodal vs. sham) and frequency of stimulation (0.8 vs. 5 Hz).We found a significant relative enhancement of the delta activity after the anodal tDCS at 5 Hz compared to that at 0.8 Hz. This increase, even though not reaching the statistical significance compared to sham, is concomitant to a significant increase of subjective sleepiness, as assessed by a visual analog scale. These two phenomena are linearly related with a regional specificity, correlations being restricted to cortical areas perifocal to the stimulation site.We have shown that a frontal oscillating anodal tDCS at 5 Hz results in an effective change of both subjective sleepiness and spontaneous slow-frequency EEG activity. These changes are critically associated to both stimulation polarity (anodal) and frequency (5 Hz). However, evidence of frequency-dependence seems more unequivocal than evidence of polarity-dependence.

Three-dimensional human somatosensory evoked potentials

Median nerve somatosensory evoked potentials were recorded from 30 normal adults using conventional scalp derivations and an orthogonal bipolar surface electrode montage. This allowed the determination of the spatial orientation of the hypothetical centrally located equivalent dipole derived from the evoked response recorded in 3-dimensional voltage space. The 3-dimensional voltage trajectory describing changes in equivalent dipole orientation and magnitude revealed 4 major apices between 5 and 25 msec, 3 of which corresponded to the traditional P14, N20 and P25 peaks. A fourth apex at 17 msec was not as evident in the conventional recordings and signaled a transition from a vertical P14–N18 generator process to a horizontal N20 generator process. The normal within- and between-subject variability of trajectory apices, segments and planes are described, along with the theoretical and practical implications of this recording technique.

Somatosensory evoked potentials via lumbosacral pathways: A comment on naturalistic stimulation

Somatosensory evoked potentials (SEPs) were averaged from several scalp derivations in 3 subjects (1 M, 2 F) for transcutaneous electrical stimulation of toes and perineum. These lumbosacral SEPs were amplitude-focal at vertex for 40–300 msec components. Peak latencies in the 40–120 msec time segment were significantly later for perineal stimulation of equivalent subjective magnitude, contrary to expectation from relative afferent pathway lengths. One subject (M) showed a monotonic latency retardation of an intermediate component (SP50-SN75) for equivalent stimuli at lips, fingers, toes, genitalia, and perianum. Vertex potentials (SN140-SP200) did not show this temporal sequence. Though interpretation of such data is currently ambiguous, systematic application of comparative evoked potential/magnetic field techniques could yield data useful to clinical neurologists.

Evoked-potential correlates of delayed letter-matching

Averaged evoked potentials (AEPs) were recorded from 11 scalp derivations in subjects participating in a delayed letter-matching task. A trial involved presenting a sequence of random dot displays (20 msec in duration), then a letter, then a second series of random dot displays, followed by a second letter. The match and mismatch conditions were equally probable and letter content was counterbalanced across trials. The number of illuminated dots and the retinal area subtended were equal for all displays. AEPs elicited by the random dot displays preceding the first letter differed from AEPs elicited by the random dots following the first letter. Significant differences were also noted between match and mismatch AEPs. The differences occurred primarily between 300 and 400 msec and were maximal in occipital, parietal, and posterior temporal derivations.

An analytic case study of periparoxysmal events in an implanted temporal lobe epileptic

A study of periparoxysmal events recorded from depth structures with implanted electrodes in a temporal lobe epileptic were undertaken with the methods of spectral and discriminant analysis. The study is based on (1) Eleven samples of poly-sharp depth activity (without simultaneous surface records) and associated periparoxysmal samples, and (2) 11 samples of depth paroxysmal delta (with surface recordings) and associated periparoxysmal epochs. The EEG samples were grouped according to (1) records taken from a maintained (10 sec) paroxysmal event (mp); (2) preparoxysmal samples (pp) taken just prior to mp; and (3) records at least 20 sec removed from mp, identified as the steady state or ss period. The objective of the study was to identify consistent features in these groups of EEG records as well as to demonstrate adequate discrimination (or the lack of it) in the 3 following situations: (1) Steady state/preparoxysmal discrimination (in order to identify changes premonitory to the mp); (2) an mp/pp discrimination for left sided sites when the right side was undergoing a well developed paroxysmal event (in order to assess the role of the ‘unaffected’ hemisphere during seizures on the right); and (3) classification and discrimination of two groups of surface records on the basis of paroxysmal events in the depth (with and without use of depth and depth × surface information). The results indicate that the ss/pp discrimination of depth records can be reliably (85%–95% correct) achieved. This, in turn, qualifies the pp period as distinct from the steady state. The variates of most importance in this discrimination indicate that the pp state is, at least in part, composed of features which will later be seen in the paroxysmal episode. There do exist, however, other features in the pp group which are not conspicuous in the mp. The study of left sided sites in the presence and absence of seizures on the right indicates that left sided records from these two states cannot be classified with error rates of less than approximately 20%–30%. The most impressive class of variates is the loss of L × R coherence when the right side is in a paroxysmal period. Thus, the primary does not appear to drive its left sided homology. The difficulty of the discrimination implies that left sided activity doesn't change dramatically in the presence of seizures on the right. It is suggested that such a phenomenon may be used to predict recrudescence of symptons after removal of the primary. Lastly, we consider the classification of surface records in the presence and absence of depth seizures. Error rates for this discrimination showed rather different results depending on the estimator used. We prefer the more conservative figure of about 25% misclassification for the steady state group. The most important variate was loss of coherence among the overlying scalp derivations in the same frequency band as the underlying seizure.

Reponses aux stimulations visuelles dans les etats de coma aigu et de coma chronique

The visual evoked responses (VER) recorded from the midline in bipolar derivations (frontovertex and vertex-occiput) have been averaged in the Mnemotron CAT 400 C in patients in coma dépassé (irreversible coma), in deep acute coma and in chronic coma (41 subjects in all) during EEG investigations. These studies consisted of sequences of 40 flashes repeated every 5 min and for 1–1.5 h in each patient. 1. 1. In coma dépassé (Stage IV in Fischgold and Mathis) the only response which can be averaged is the electroretinogram (ERG). 2. 2. In acute coma (Stage III of Fischgold and Mathis) with a rapidly fatal outcome we find: (a) stereotyping and areactivity of the occipital responses during the same examination; (b) earlier deterioration of the cortical VER than of the spontaneous activity and, in the terminal phase, its extinction always preceding that of the EEG; (c) contamination of the scalp derivations by the ERG, clearly increasing during electro-clinical worsering. 3. 3. In comas which have become chronic: (a) the VER, extremely reduced in the initial stages, can show later recovery, to become fixed, during the third period, with an unchanging stereotyped appearance; (b) during this third period the differentiation between the responses during waking and sleeping become more and more clear. With the eyes open the VER and ERG are either non-existant or considerably reduced, contrasting with the exaggerated photopalpebral reflex. During sleep the photopalpebral reflex is abolished, the ERG increased and the VER facilitated, possibly with a rhythmic after-discharge; (c) stereotyping of the VER is thus maintained in chronic coma but with two modes of expression, one belonging to the state of waking, the other seen during sleep. This differentiation between waking and sleeping is one of the characteristics of passing into the chronic state; (d) there is an over-all agreement between the appearance of the spontaneous EEG and the cortical evoked responses but disagreement appears when the various elements are examined in detail.